linux-kernel - [PATCH] printk: fix the check on modifying printk

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Date:   Thu, 17 Aug 2023 07:47:45 +0800
From:   Yun Zhou <yun.zhou@...driver.com>
To:     <pmladek@...e.com>, <senozhatsky@...omium.org>,
        <rostedt@...dmis.org>, <john.ogness@...utronix.de>
CC:     <linux-kernel@...r.kernel.org>
Subject: [PATCH] printk: fix the check on modifying printk_devkmsg

When we use 'echo' to write a string to printk_devkmsg, it writes '\0' at
the end. It means lenp is larger then the length of string 1. However,
sometimes we write data with string length by other way, e.g
    write(fd, string, strlen(string));
In this case, the writing will fail.

Comparing err with string length is able to handle all scenarios.

Signed-off-by: Yun Zhou <yun.zhou@...driver.com>
---
 kernel/printk/printk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 357a4d18f6387..992872f7b7747 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -229,7 +229,7 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
 		 * Do not accept an unknown string OR a known string with
 		 * trailing crap...
 		 */
-		if (err < 0 || (err + 1 != *lenp)) {
+		if (err != strlen(devkmsg_log_str)) {
 
 			/* ... and restore old setting. */
 			devkmsg_log = old;
-- 
2.27.0